In monochrome half-tone image reproduction, various shades appearing in a continuous tone original image are reproduced on paper by a single tone ink. To simulate the multiple tones in the original, the continuous tone image is converted into a half-tone image comprising a plurality of different size dots of a uniform optical is density level, as is well known in the art. Also, as is well known in the art, color images are printed using 4-color separations, where each pixel of a digital image corresponds to a combination of color and intensity represented on each of the 4 color separations. Thus, each pixel in the digital image corresponding to one of these separations is then translated into a specified dot size on a printing plate to be used for that color. The conversion of color intensity to dot size is typically represented by a generic transfer curve that takes the input of desired color tone and converts it to desired dot size in the imagesetter. Such a transfer curve may be linear (i.e. a 20% color tone equates to a dot that is 20% of the maximum dot size, and a 70% color tone equates to a dot that is 70% of the maximum dot size), or may be a curve that corresponds to some other mathematical function.
Printing of digital images is typically complicated by the phenomenon of “dot gain,” in which the size of printed halftone dots is larger than that specified by the digital image file employed by the imagesetter. Dot gain interferes with the ability to produce similar standardized colors between printing press runs. To compensate for this effect, historically, printing press users entered in specific X and Y transfer coordinates, representing on the X-axis the desired color density, expressed as a percentage of maximum color tone, and on the Y-axis, the actual color density setting (i.e. printing plate dot size) used to produce the desired color output, as depicted in FIG. 8. This practice evolved into the generation and application of press adjustment transfer curves by mathematically fitting the curve to a set of experimentally derived points, as shown in FIG. 9. Each adjustment curve is a mathematical function specifying the required dot size on the printing plate to compensate for on-press dot gain, thus resulting in a calibrated plate. The adjustment curve thus represents mapping of an input color percentage to a desired output dot size on the printing plate to produce the desired result, taking into account dot gain. Between press runs, there may be a great deal of variation. These variations depend on numerous press factors (press type, ink type, speed, paper type) as well environmental factors (temperature, humidity, etc.). As some of these conditions may change during or between press runs, the ideal press adjustment curve may change as well.
Traditionally, little has been done to correct dot gain in offset printing, because of the limited dot gain that occurs. As new printing methods have become the industry standard, however, dot gain has become an increasingly larger problem. For example, dot gain in flexographic printing may be much greater than in offset printing, requiring the use of press adjustment curves. Even in offset printing, the use of new screening technologies such stochastic screening and concentric screening have necessitated the use of adjustment curves. In many cases, users create and apply new custom curves whenever the printed image begins to deteriorate. Historically, users created custom curves for each press. These custom curves could vary from user to user and were often based on time-consuming and expensive test runs. The number of custom curves created by users has often become unmanageable, because of the number of factors and variables involved. Creation of numerous custom curves may lead to more costly, time-consuming, and laborious adjustments if the wrong curve is applied. Because it may cost thousands of dollars to create a standard press run, it can be cost- and time-prohibitive to perform numerous standard press runs.
Therefore, a system is needed that can quickly take the information from a current press run and determine the optimum adjustment curve. A system is also needed that can bring organization to the curve selection process, instead of the chaos of numerous unsynchronized custom curves.